View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Directory of Open Access Journals Processing and Application of Ceramics 8 [2] (2014) 53–57 DOI: 10.2298/PAC1402053G Processing and characterization of CaTiO3 perovskite ceramics Guilherme Gralik1,∗, Alessandra E. Thomsen2, Cristian A. Moraes1, Fabiano Raupp-Pereira1, Dachamir Hotza2 1Department of Mechanical Engineering (EMC), Federal University of Santa Catarina (UFSC), Florianópolis, SC, Brazil 2Department of Chemical Engineering (EQA), Federal University of Santa Catarina (UFSC), Florianópolis, SC, Brazil Received 20 February 2014; Received in revised form 2 April 2014; Received in revised form 1 May 2014; Accepted 15 May 2014 Abstract Calcium titanate (CaTiO3) ceramics with perovskite structure were produced by solid state reaction. Calcium carbonate (CaCO3) and titanium dioxide (TiO2) were mixed (in molar ratios 1/1 and 3/2), and the obtained mixtures were calcined at 1150°C in successive thermal cycles. The obtained samples were characterized by differential thermal analysis, thermogravimetry, X-ray diffraction, measurement of particle size distribution and linear thermal shrinkage. XRD results indicated that the samples have perovskite CaTiO3 structure with small amount of secondary CaO and TiO2 phases, and their phase composition depends on the heat treatment conditions. The measured values of electrical resistivity were within the characteristic range of insulating materials and approach values corresponding to semiconducting ceramics. Keywords: CaTiO3, structural characterization, conductivity, thermistor I. Introduction coefficient (NTC), and is a refractory material with high resistance to caustic corrosion [6]. Thermistors are a Electroceramics have numerous applications due to type of resistors that change their electrical resistance their specific structures and physical properties, such as depending on temperature. The resistance can be in- interconnect, packaging and substrates materials for mi- creased or decreased with temperature depending on croelectronics or as individual circuit components, par- the carrier mobility. Thermistors that exhibit a decrease ticularly as capacitors or sensors [1–3]. in resistance with increasing temperature are called Calcium titanate (CaTiO3) is a ceramic material with negative temperature coefficient (NTC) thermistors and a perovskite structure. This compound does not show those that exhibit an increased resistance with higher anisotropy, and its structure is cubic above 1307 °C, temperatures are known as positive temperature coeffi- tetragonal between 1107 and 1227°C and orthorhom- cient (PTC) thermistors. Thermistors can be classified bic below 1107 °C [4]. The cubic perovskite structure according to electrical relationships, such as current- can be obtained at room temperature when doped with time, voltage-current and resistance-temperature [7]. cation receptors. CaTiO3 shows distinct structural, elec- The oxides with perovskite structure can be obtained trical and optical properties and, therefore, is of great by various methods of synthesis, particularly by solid scientific and technological interest [5]. Calcium ti- state reactions (mixed oxides) or wet-chemical prepa- tanate is also used as a resistor element with thermal ration (sol-gel, Pechini). The mixed oxide method is sensitivity (thermistor), due to its negative temperature the reaction between oxides and/or carbonates, follow- ing milling, mixing and heat treatment at relatively high ∗ Corresponding author: tel: +55 48 37214004 temperatures close to the melting points of compo- e-mail: [email protected] (G. Gralik) e-mail: [email protected] (A.E. Thomsen) nents [8]. The method is widely used due to low cost, e-mail: [email protected] (C.A. Moraes) easy preparation and large amount of material produced, e-mail: [email protected] (F. Raupp-Pereira) when compared with wet-chemical techniques. How- e-mail: [email protected] (D. Hotza) ever, some drawbacks are firing at high temperatures, 53 G. Gralik et al. / Processing and Application of Ceramics 8 [2] (2014) 53–57 products with multiphase character, wide particle size tions at 1150°C. The thermal treatment was performed distribution, loss of stoichiometry due to the volatiliza- in three successive cycles (Fig. 2) with a heating rate of tion of reactants at elevated temperatures and the pres- 10°C/min and 2 h holding times in a resistive furnace ence of contaminants acquired during the grinding pro- (Jung J200). Between the calcination steps, the powders cess. Thus, the reproducibility of the method is difficult were comminuted manually in an agate mortar, mini- [9]. mizing possible stoichiometric differences between the The objective of this study is to evaluate the influence surface and the bulk of the particles. of heat treatments on the electrical properties of CaTiO3 obtained by the mixed oxides method from two different initial compositions. II. Materials and Methods Calcium titanate powders were prepared from cal- cium carbonate (CaCO3) and titanium dioxide (TiO2)by solid state reactions under controlled conditions. Two different compositions (M1 and M2) were selected in accordance with the CaO-TiO2 phase diagram (Fig. 1). The phase diagram of the CaO–TiO2 system predicts the existence of three stable compounds: Ca3Ti2O7, Ca4Ti3O10 and CaTiO3. The first two peritectic decom- position reactions should occur around 1750°C and 1840 °C, respectively. The third compound, CaTiO3, is formed congruently at 1960°C with a eutectic at Figure 2. Experimental calcination cycles in successive stages 1450°C, between CaTiO3 and TiO2 [10]. Although ther- followed by natural cooling and deagglomeration modynamic equilibrium under normal industrial ope- in agate mortar rating conditions is normally not reached, the phase di- agram was used to predict adequate compositions and The particle size distribution of the precursor mix- processing parameters. tures M1 and M2 was measured with PTD Master Sizer 2000, Malvern. Linear thermal shrinkage was measured with an optical dilatometer (Misura) and the obtained data were used for optimizing the thermal treatment conditions. Differential thermal analysis and thermo- gravimetry (DTA/TG, EP STA 409, Netzsch) were car- ried out at 10°C/min in an oxidizing atmosphere. Inves- tigation of the crystalline phases present after every cal- cination step was performed on Philips XRD, PW 3710 using Ni-filtered CuKα radiation. Electrical conductivity measurements were carried out using cylindrical samples of the powders pressed in a manual hydraulic press at 100MPa, with a 22mm diameter and thickness between 1 and 3mm. The elec- trical conductivity was measured at room temperature by the standard two-point method, using a DC power source (Keithley, 6220). The potential difference was measured with a voltmeter (Keithley, 6517A). The cal- culation of the resistivity was performed according to the following equation [11]: d2π V 1 ρ = · · (1) Figure 1. Location of compositions M1 and M2 in the 4 I L diagram of the binary system CaO-TiO [10] 2 where ρ is the resistivity (Ω·m); d, the diameter of the sample (mm); V, the potential difference (V); I, the cur- Two samples with the compositions M1 (51.9wt.% rent (A); and L, the sample thickness (mm). CaCO3 and 48.1wt.% TiO2) and M2 (41.1wt.% CaCO3 and 58.9wt.% TiO2) were prepared by mixing of III. Results and Discussion CaCO3 (Vetec) and TiO2 (Aldrich) in dry ball mill jar over a period of 24h. The obtained mixtures were trans- Figure 3 shows the particle size distribution of the formed in calcium titanate powders by solid state reac- precursor powders M1 and M2. It can be seen that the 54 G. Gralik et al. / Processing and Application of Ceramics 8 [2] (2014) 53–57 ric curves for both precursor mixtures. Softening of the sample M1 begins at ∼1150°C with shrinkage of ∼33%, whereas shrinkage of the sample M2 starts at ∼1050°C. The results of differential thermal analysis (DTA/DTG) and thermogravimetry (TG) for the sample M1 are presented in Fig. 5. The formation of titanate takes place at temperatures above 500°C and the weight loss of the compound develops up to ∼800°C. Thus, the DTA exothermic peak between 650 and 800°C refers to the carbonate decomposition and formation of titanate. Above 800°C there is a slight weight loss as a result of the continuing reaction of residual carbonate and the formation of calcium titanate. Figure 6 shows the XRD patterns of the samples M1 and M2, after three different thermal cycles (C1 to C3). The dominant XRD peaks belong to perovskite calcium Figure 3. Particle size distribution titanate (CaTiO3) phase. However, peaks of minor in- tensity corresponding to the precursor phases TiO2 and CaO are still observed even after 3 cycles of thermal treatment. The intensity of diffraction peaks of the sam- ples M1 and M2 are different. It seems that the sample M1 with molar ratio Ca/Ti = 1 has smaller amount of undesirable secondary phases (CaO and TiO2). Micrographs of the samples calcined at 1150°C in three cycles (M1-C3 and M2-C3) are shown in Fig. 7. The fracture surfaces have some interconnected poros- ity and the grain sizes around 1 µm, which are within the range found in the literature [12]. The microstruc- tures are similar, but the pellet M2-C3 presents a greater variation in grain size in relation to the sample M1-C3. Table 1 presents the resistivity and conductivity val- ues measured after each calcination cycle (C1, C2, C3) for the pressed pellets M1 and M2. The electrical prop- erties are also dependent on the Ca/Ti ratio. The pel- Figure 4. Linear thermal shrinkage of M1 and M2 samples let M1 has a lower resistivity compared to M2 for all heat treatments considered. Calcination led to decreased particle size distribution is bimodal with average par- resistivity (increased conductivity) of the M1samples. ticle sizes (d50) of 10 µm and 8.4 µm for the sample However, for M2 samples the subsequent calcination M1 and M2, respectively. Figure 4 shows dilatomet- did not show a linear relationship with the conductivity; (a) (b) Figure 5.